My six year old boy is fascinated when he see's me meditating sometimes and having conversations with my wife on Buddhist related topics.
So i have told him the basics of Kamma, in the aspect that if they are rude it will not only hurt others feelings, but make them feel bad for hurting them too, and other cause and effect topics, also getting him to sit down and try to have very short meditation sessions, foccusing on the breath at the nostrils.
He is very keen to learn more always...as am i. that seems a good start for me anyway, hope it helps!?
"Happy, at rest, may all beings be happy at heart. Whatever beings there may be, weak or strong, without exception, long, large, middling, short, subtle, blatant, seen & unseen, near & far, born & seeking birth: May all beings be happy at heart."